1. Field of the Invention
This invention generally relates to duplexers, and more particularly, to a duplexer in which ladder type filters are for use in a transmit filter and a receive filter.
2. Description of the Related Art
In recent years, with the advancement of mobile communications systems, portable information terminals or the like are rapidly wide spreading. Developments have been done to reduce the sizes of the above-described terminals and to improve performance thereof. Both analogue and digital are used for mobile telephone systems, in which 800 MHz to 1 GHz frequency bandwidth and 1.5 GHz to 2.0 GHz frequency bandwidth are mainly used. There have been proposed antenna duplexers that employ a surface acoustic wave filter or piezoelectric thin-film resonator filter so as to be employed in the devices for mobile communications systems.
In the development of the mobile telephones these days, the performance of the terminals is improved by variations of the systems, for example, by utilizing a dual mode or dual band. In the dual mode, the mobile telephones operate on both analogue (Code Division Multiple Access: CDMA) and digital (Time Division Multiple Access: TDMA) networks. In the dual band, 800 MHz bandwidth and 1.9 GHz bandwidth or 900 MHz bandwidth and 1.5 GHz bandwidth are used at the same time. Hence, there is a demand for high-performance parts (filters) for use in the above-described terminals.
Meanwhile, in addition to the improvement of the performance, there are also demands for size reduction and cost reduction. A number of antenna duplexers for use in high-performance terminals are composed of a combined duplexer in which a dielectric material is used for the transmit filter and received filter, a dielectric material is used for at least one of the transmit filter and the receive filter and a surface acoustic wave device is used for the other, or only surface acoustic wave devices are used.
The dielectric duplexer has a large size, making it difficult to reduce the size of the portable terminal device or to reduce the thickness thereof. Even when the surface acoustic wave device is used for one of the transmit filter and the receive filter, the size of the dielectric device makes it difficult to reduce the size of the portable terminal device or to reduce the thickness thereof. The duplexer device having a conventional surface acoustic wave filter used therein has: a module type device in which there are independently provided a transmit filter, a receive filter, and a matching circuit on a printed board; and a monolithic type device in which there are mounted a transmit/receive filter chip on a multilayer ceramic package and a matching circuit provided therein. The volume of the afore-described duplexer device has a volume approximately ⅓ to 1/15 that of a dielectric duplexer. With respect to only the height, the thickness can be reduced by approximately ½ to ⅓.
Next, a commonly used duplexer will be described. FIG. 1 is a block diagram of a duplexer. FIG. 2 shows bandpass intensity with respect to frequency of the duplexer. Referring now to FIG. 1, the duplexer includes: two filters of, for example, a transmit filter 10; a receive filter 12; an impedance matching circuit (hereinafter, simply referred to as matching circuit) 14; a common terminal Ant; and respective terminals, for example, a transmitting terminal Tx and a receiving terminal Rx.
The common terminal Ant is a terminal coupled to an external circuit that sends and receives radio waves through an antenna. The transmitting terminal Tx is a terminal coupled to an external transmitting circuit to input a signal having a desired center frequency. The receiving terminal Rx is a terminal connected to an external receiving circuit to output a signal having a desired center frequency. Another terminal, not shown, other than the transmitting terminal Tx and the receiving terminal Rx is connected to the ground level (GND).
In general, the transmit filter 10, the receive filter 12, and the matching circuit 14 are housed in a multilayer ceramic package. The transmit filter 10 and the receive filter 12 are respectively composed of, for example, a surface acoustic wave filter or piezoelectric thin-film resonator filter, and respectively have different center frequencies F1 and F2. Here, in most cases, F2>F1 is satisfied. For example, in a duplexer for a W-CDMA system operating at 2 GHz, the transmit band ranges 1920 MHz to 1980 MHz and the receive band ranges 2110 MHz to 2170 MHz. The difference between the transmit band and the receive band is 130 MHz.
The matching circuit 14 is provided so that the transmit filter 10 and the receive filter 12 do not degrade each other. Z1 represents a characteristic impedance when the transmit filter 10 is viewed from the common terminal Ant, and Z2 represents a characteristic impedance when the receive filter 12 is viewed therefrom. When F1 is the frequency of the signal fed from the common terminal Ant, the characteristic impedance Z1 at the side of the receive filter 12 corresponds to that of the common terminal Ant, and the characteristic impedance at the side of the receive filter 12 is infinite and the reflection coefficient is 1, by means of the effect of the matching circuit 14. The characteristic impedance Z2 of the receive filter 12 is configured to correspond to that of the common terminal Ant.
As disclosed in Japanese Patent Application Publication No. 2004-328676, there is provided a duplexer in which the transmit filter 10 and the receive filter 12 are mounted in a stacked package and a ground line pattern that forms an inductance is provided on a die attach layer and a lower layer thereof in the stacked package, for the purpose of size reduction of the duplexer.
There is a demand for duplexers to improve the attenuation characteristic in the opposite passband (suppression), in other words, to improve the attenuation characteristic in the transmit band of the receive filter and to improve the attenuation characteristic in the receive band of the transmit filter. In particular, it is difficult to improve the attenuation characteristic of the opposite passband in the duplexer for use in the W-CDMA system operating at 2 GHz.